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<a name="boost.proto.switch_"></a><div class="titlepage"></div>
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<h2><span class="refentrytitle">Struct template switch_</span></h2>
<p>boost::proto::switch_ &#8212; For matching one of a set of alternate grammars, which are looked up based on
          the result type of the transform passed in second template parameter. 
          If no transform is passed, the default one is <code class="computeroutput"><a class="link" href="tag_of.html" title="Struct template tag_of">proto::tag_of</a>&lt;<a class="link" href="_.html" title="Struct _">proto::_</a>&gt;()</code>
          so the default matching is based on the expression's tag type. When used as a transform,
          <code class="computeroutput">proto::switch_&lt;&gt;</code> applies the transform associated
          with the sub-grammar that matches the expression.</p>
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<h2 xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv-title">Synopsis</h2>
<div xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="refsynopsisdiv"><pre class="synopsis"><span class="comment">// In header: &lt;<a class="link" href="../../proto/reference.html#header.boost.proto.matches_hpp" title="Header &lt;boost/proto/matches.hpp&gt;">boost/proto/matches.hpp</a>&gt;

</span><span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> Cases<span class="special">,</span> <span class="keyword">typename</span> <a class="link" href="../../Transform.html" title="Concept Transform">Transform</a><span class="special">&gt;</span> 
<span class="keyword">struct</span> <a class="link" href="switch_.html" title="Struct template switch_">switch_</a> <span class="special">:</span> <span class="keyword"></span> <a class="link" href="transform.html" title="Struct template transform">proto::transform</a><span class="special">&lt;</span><span class="identifier">switch_</span><span class="special">&lt;</span><span class="identifier">Cases</span><span class="special">,</span> <span class="identifier">Transform</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="special">{</span>
  <span class="comment">// types</span>
  <span class="keyword">typedef</span> <span class="identifier">switch_</span> <a name="boost.proto.switch_.proto_grammar"></a><span class="identifier">proto_grammar</span><span class="special">;</span>

  <span class="comment">// member classes/structs/unions</span>
  <span class="keyword">template</span><span class="special">&lt;</span><span class="keyword">typename</span> <a class="link" href="../../Expr.html" title="Concept Expr">Expr</a><span class="special">,</span> <span class="keyword">typename</span> State<span class="special">,</span> <span class="keyword">typename</span> Data<span class="special">&gt;</span> 
  <span class="keyword">struct</span> <a class="link" href="switch_/impl.html" title="Struct template impl">impl</a> <span class="special">:</span> <span class="keyword"></span> 
    <span class="identifier">Cases</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">case_</span><span class="special">&lt;</span>
      <span class="keyword">typename</span> <a class="link" href="when.html" title="Struct template when">when</a><span class="special">&lt;</span><span class="identifier">_</span><span class="special">,</span> <span class="identifier">Transform</span><span class="special">&gt;</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">impl</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">Data</span><span class="special">&gt;</span><span class="special">::</span><span class="identifier">result_type</span> 
    <span class="special">&gt;</span><span class="special">::</span><span class="keyword">template</span> <span class="identifier">impl</span><span class="special">&lt;</span><span class="identifier">Expr</span><span class="special">,</span> <span class="identifier">State</span><span class="special">,</span> <span class="identifier">Data</span><span class="special">&gt;</span>
  <span class="special">{</span>
  <span class="special">}</span><span class="special">;</span>
<span class="special">}</span><span class="special">;</span></pre></div>
<div class="refsect1">
<a name="idp136789168"></a><h2>Description</h2>
<p>
          An expression type <code class="computeroutput">E</code> matches
            <code class="computeroutput">proto::switch_&lt;C,T&gt;</code> if
          <code class="computeroutput">E</code> matches
            <code class="computeroutput">C::case_&lt;boost::result_of&lt;<a class="link" href="when.html" title="Struct template when">proto::when</a>&lt;<a class="link" href="_.html" title="Struct _">proto::_</a>,T&gt;(E)&gt;::type&gt;</code>.
          </p>
<p>
          When applying <code class="computeroutput">proto::switch_&lt;C,T&gt;</code> as a
            transform with an expression <code class="computeroutput">e</code> of type
            <code class="computeroutput">E</code>, state <code class="computeroutput">s</code> of
            type <code class="computeroutput">S</code> and data <code class="computeroutput">d</code>
            of type <code class="computeroutput">D</code>, it is equivalent to
            <code class="computeroutput">C::case_&lt;boost::result_of&lt;<a class="link" href="when.html" title="Struct template when">proto::when</a>&lt;<a class="link" href="_.html" title="Struct _">proto::_</a>,T&gt;(E,S,D)&gt;::type&gt;()(e, s, d)</code>.
          </p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2008 Eric Niebler<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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